The present invention relates to a system for controlling a weld-current in a direct current arc welding apparatus for short arc welding. Short arc welding is a metal transfer mode in gas metal arc welding where a drop of molten metal will dip into a weld pool before detachment from a consumable electrode in a process of metal transfer.
In gas metal arc welding a consumable electrode is continuously fed towards a welding zone. In general a system for metal arc welding includes a welding torch, a work piece, a power source, a wire feed unit and a shielding gas supply. A system for metal arc welding furthermore includes a system for controlling a weld-current of the power source. The system for controlling the weld-current is provided to generate a reference current to the power source. The reference current controls the power source such that melting of the wire electrode and transfer of the molten metal to the work piece may be controlled as desired. The weld-current and voltage are controlled to ensure that the welding may be performed in a desired metal transfer mode.
In Gas Metal Arc Welding (GMAW), including both Metal Active Gas welding (MAG) and Metal Inert Gas (MIG) welding, an electric arc is established between the work piece and the consumable wire electrode. The arc continuously melts the wire as it is fed to the weld puddle. The arc and the molten material are shielded from the atmosphere by a flow of an inert gas, or an active gas mixture. The MIG and MAG welding processes operate on D.C. (direct current) usually with the wire electrode positive. This is known as “reverse” polarity. “Straight” polarity, is less frequently used because of poor transfer of molten metal from the wire electrode to the work piece. Weld-currents of from 50 amperes up to more than 600 amperes are commonly used at welding voltages of 15V to 32V. A stable, self correcting arc may be obtained by using a constant voltage power source and a constant wire feed speed. Continuing developments have made the MIG process applicable to the welding of all commercially important metals such as steel, aluminum, stainless steel, copper and several others.
The MIG and MAG welding processes provide many advantages in manual and automatic metal joining for both low and high production applications. Its combined advantages when compared to manual metal arc welding (MMA) submerged arc (SAW), and tungsten inert gas (TIG) are:    1) Welding is possible in all positions.    2) No slag removal required.    3) High weld metal deposition rate.    4) Overall times for weld completion about ½ that of covered electrode.    5) High welding speed. Less distortion of the work piece.    6) High weld quality.    7) Large gaps filled or bridged easily, making certain kinds of repair welding more efficient.    8) No stub loss as with stick electrode
In MIG and MAG welding technologies distinction is made between different metal transfer modes: short arc transfer mode, mixed arc (globular) transfer mode, spray arc transfer mode, and pulsed spray arc transfer mode.
During short arc transfer mode fairly large molten droplets are created. The molten droplet will grow to a state where it bridges a gap between the electrode and a weld pool. A short-circuit of the power source and an extinction of the arc will take place momentarily. A pinch effect will be controlled by a ramp generator to complete the transfer of the molten droplet to the weld pool. Short arc welding is performed at a relatively low voltage and weld-current.
As the weld-current and voltage are increased above the maximum recommended for short arc welding, mixed arc transfer occurs. The droplets, which may vary in size, are made up of a mixture of short-circuiting and non-circuiting droplets. This mode of metal transfer can be erratic, with spatter and fumes being common.
In spray arc welding mode small molten droplets are randomly transferred from the electrode. The small molten droplets do not short-circuit the arc. In spray arc welding the arc is stable and no troublesome spatter is produced.
In pulsed spray arc welding mode small molten droplets are transferred, one by one by control of a pulsed weld-current, from the electrode without short-circuiting the arc. Pulsed arc welding requires complex and expensive welding apparatuses.
Short-arc welding is performed at a relatively low voltage and weld-current. The feeding speed of the wire is adapted to the weld-current such that a droplet of molten metal is transferred to the work piece at contact with the work piece. The size of the droplet should be such that splatter generally is avoided.
Short-arc welding normally uses small wire in the range of 0.030 in. (0.76 mm) to 0.045 in. (1.1 mm) diameter and operates at low arc lengths (low voltages) and weld-currents. A small, fast solidifying weld puddle is obtained. This welding technique is particularly useful for joining thin materials in any position, thick materials in the vertical and overhead position, and for filling large gaps. Short arc welding should also be used where minimum distortion of the work piece is a requirement. Metal is transferred from the wire to the weld pool only when contact between the two is made, or at each short circuit. The wire short circuits to the work piece 20 to 200 times per second.
FIG. 1 schematically illustrates one complete ideal short arc cycle. A droplet of molten material develops at the end of the electrode. When the droplet touches the weld pool (A) the arc short-circuits, weld-current begins to rise and the droplet is transferred. Thereafter the arc is reignited. The wire is fed with a velocity allowing contact between the droplet and the weld pool to take place before detachment of the droplet. During contact between the electrode and the weld pool via the droplet, the arc will be extinguished by another short (I). Thereafter the cycle begins again. There is no metal transferred during the arcing period; only at the short circuits. The cycle is ideal in the sense that no so-called open circuit state occurs.
An open circuit state is a state when there is neither an arc nor short circuit at the electrode. The open circuit state has a higher voltage between the electrode and the work piece in comparison to the arc state. High voltage at the open circuit state enables a more rapid transition from short circuit state to arc state.
The short arc welding process is by its nature stochastic and turbulent. The control of the welding process in short circuit phase and arc phase is challenging, and the occurrence of open circuit state adds to the complexity of controlling the short arc welding process. Open circuit state is normally undesired and has a random appearance.
In the voltage feedback loop, the voltage between the electrode and the welding pool is measured. The measured voltage is compared to a reference voltage. The current regulator regulates an output current in dependence of the difference between the sensed voltage and the reference voltage in order to reduce the regulation error in a conventional manner. Suitably a PI-regulator may be used for this purpose.
In prior art systems for controlling short arc welding, the arc phase is usually voltage controlled by a voltage regulator connected in series with a current rise limiter. This leads to difficulty of control of the arc and short circuit state since the voltage regulator will have an influence on the current raise limiter and vice versa.